Materials, such as fluff, padding, paper, board, and tissue are often wound into a roll and then stored for subsequent processing operations. During the subsequent processing operation, the roll is unwound and the sheet material is run through another machine for further processing steps. A common unwind used for rolls of soft, compressive, and relatively weak materials, such as facial tissue, bath tissue, paper toweling and the like, uses one or more belts that contact at least a portion of the roll's periphery. These unwinds are commonly referred to as a belt-driven unwind. The surface contact between the belt(s) and the roll transmits the drive force needed to accelerate, decelerate, and rotate the roll. The belt(s) are driven by a drive roller connected to a power source, such as a drive motor, that accelerates, decelerates, or rotates the belt(s) that are wrapped around at least a portion of the drive roller's surface. In order to ensure proper belt tracking and to prevent slippage of the belt(s) at the drive roller, the belt tension must be kept at a higher level for proper operation of the belt-driven unwind.
For some tissue materials, a belt-driven unwind is not suitable since the belt's pressure against the outer surface of the roll can cause grooves to appear in the roll, thereby damaging the underlying tissue. Such damage is more common with high bulk, soft tissue products used by individual consumers as opposed to lower bulk tissue products commonly sold to the service and industrial markets. The pressure of the belt(s) against the roll occurs since the belt tension can only be reduced to a minimum value before belt tracking and slippage of the belt(s) prevent further reductions in the belt tension. Additionally, to prevent slippage of the roll at the roll/belt interface, the belts must be loaded against the roll's surface with sufficient force to generate the drive forces needed. Often the belts are wrapped around a significant portion of the roll's periphery. These factors contribute to a minimum pressure for the belt(s) against the surface of the roll that cannot be reduced without creating runnability problems, i.e. belt and/or roll slippage, especially during acceleration of a maximum diameter roll. Thus, it is seen that there are conflicting requirements for belt tension. The belt needs to be tight for guiding and to transmit power to the roll, but high belt tension can damage soft, bulky rolls of material.
One means of preventing this damage is to use a center-driven unwind. One suitable center-driven unwind for soft tissue rolls is disclosed in U.S. Pat. No. 5,906,333, entitled Center Drive Unwind System and issued to Fortuna et al. on May 25, 1999. Another suitable unwind for soft tissue rolls is a combination center-driven and belt-driven unwind disclosed in U.S. Pat. No. 6,719,240, entitled System and Method for Unwinding Tissue Webs and issued to Hanson et al. on Apr. 13, 2004. Center-driven unwinds have a disadvantage in that they are generally more expensive than the belt-driven unwinds. Draw control or tension control of the sheet material can be more difficult with a center-driven unwind than with a belt-driven unwind because the rotational speed of the roll must be continually changed as the roll unwinds to maintain a fixed sheet velocity at the outside perimeter of the roll. Out-of-round rolls also experience tension variations as the rolls unwind since center-driven unwinds may not be able to adjust for diameter variations of the roll within a single revolution of the roll. Center-driven unwinds can also experience slippage at or near the core when trying to accelerate large diameter, softly wound tissue rolls since the power to turn the roll must be transmitted from the core through the roll. Therefore, what is needed is a belt-driven unwind that is suitable for use with soft, bulky materials that can replace or be used in combination with a center-driven unwind.